1. Field of the Invention
The present invention relates to electrical locks used in entry and exit passages and other access controlled areas.
2. Description of the Prior Art
Electrically controlled locks are commonly used in many security applications. The flexibility provided by an electric lock assembly allows the opening and closing of the lock to be remotely controlled from a guard station to allow limited access to a facility. Additionally, use of an electric lock assembly can allow the time of access to be controlled using a time clock mechanism, as is common in bank vaults for instance.
The two most common types of electric locks in use today are those that are magnet operated and those that are solenoid operated. The magnetically operated types use a magnetic field producing apparatus on a fixed portion of the door frame and a magnetically attracted material on the door so that when the magnetic field is energized, the door is locked due to the magnetic force and when the field is de-energized, the door is unlocked and can be freely opened. A magnetic lock therefore requires the constant use of a magnetic field and the involved heat and electrical current, to maintain the door in a locked condition. Additionally, should the power fail, the lock would be disabled and the door would be opened, allowing entry to the facility.
The other common type of electrically operated lock is a solenoid type mechanism wherein the energizing of a solenoid either engages a bolt or disengages a bolt, thereby appropriately locking or unlocking the door. One major problem with a solenoid system is that the solenoid is not suitable for continuous duty use because it will overheat and therefore become inoperable after a certain period of time, generally measured in minutes. This condition is shown in U.S. Pat. No. 4,640,108 where a time-delay relay was included to de-energize the solenoid after 1.5 seconds to preclude solenoid damage. This time period dramatically limits when the solenoid can be energized and the door can be in an open or closed position therefore effectively requiring constant attention by an operator to control the lock.
U.S. Pat. No. 4,592,453 discloses a lock assembly using a clutch to allow door knob rotation to be transmitted to a latch assembly. A motor is used to engage and disengage dogs used to control the clutch for the coaxial transmission of the knob rotation. This is a cumbersome and complicated method and requires the use of a conventional bolt for locking the door to the door jamb.